1. Field of the Invention
The present invention relates to an image forming apparatus and method, and more particularly, to a device and method for correcting non-linearity of a scanner with a variable pulse width scanning position signal.
2. Description of the Related Art
Generally, in an image forming apparatus such as a laser printer, on encountering a scanning start signal, a laser scanning unit scans a photosensitive belt to form a latent image. A developing device then supplies a liquid toner to the photosensitive belt. The developing device then develops the latent image to form a toner image. This toner image is printed on a paper sheet by transfer and fixing devices. In a process, while scanning is performed, the scanner non-linearity is corrected synchronously at the rising edge of a scanning start signal. In particular, correction of the non-linearity of the scanner comprises the steps of: detecting a predetermined portion of the photosensitive belt, generating the scanning start signal; and generating a synchronizing signal corresponding to the scanning start signal wherein the frequency of the generated synchronizing signal is used as the image input frequency of the scanner.
FIG. 1 shows a block diagram illustrating the structure of a conventional device used in correcting the non-linearities associated with a scanner. Referring to FIG. 1, a conventional device for correcting the non-linearity of a scanner comprises: a counter 100, and a converter 120. The counter counts scanning start signals and is reset by the scanning start signal; a look-up table (LUT) 110 is initiated by the scanning start signal. The LUT recognizes an output of the counter 100 as an address, and outputs the look-up table information corresponding to the address in the form of digital data. The converter 120 outputs a clock signal having a frequency corresponding to the digital data output from the LUT 110. The clock signal is input to a laser scanning unit (not shown) which performs the scanning.
In the conventional device a pattern for generating a synchronizing signal is created at a portion of a photosensitive belt. The device is operated synchronously by the scanning start signal that is generated by sensing the pattern. In such a conventional device, since the pulse width of the scanning start signal may be abnormally deformed due to vibrations from sources such as the photosensitive belt, a starting point for correcting non-linearity may be imprecisely determined. FIGS. 2A and 2B show timing diagrams illustrating the problem encountered by a conventional device for correcting the non-linearity of the scanner.
Referring to FIG. 2A, when a scanning start signal (SOS) of a normal pulse width is generated, the counter 100 is reset at the falling edge of the scanning start signal, and generates a corresponding address signal, for example, address 1, address 2, address 3, etc. The corresponding digital data such as LUT information 1, LUT information 2, LUT information 3, stored in the LUT 110 is output in correspondence with the address signal. The converter 120 outputs a synchronizing signal having frequencies such as f.sub.1, f.sub.2, f.sub.3, etc., corresponding to the digital data. As the counter 100 outputs subsequent address signals, the above procedure is repeated.
However, as shown in FIG. 2B, the address signal output from the counter 100, the digital data output from the LUT 110, and the synchronizing signal output from the converter 120 are delayed when a scanning start signal (SOS.sub.max) with a larger pulse width is generated because of the vibrations in the photosensitive belt.
As described above, in the conventional device for correcting the non-linearity of the scanner, the pulse width of a scanning start signal may be abnormally narrowed or widened due to mechanical errors caused by vibrations from sources such as the photosensitive belt, and accordingly the correction starting position is imprecisely determined. Consequently, the non-linearity of the scanner cannot be corrected precisely in the conventional device.